tsan: refactor atexit handling
The current handling (manual execution of atexit callbacks)
is overly complex and leads to constant problems due to mutual ordering of callbacks.
Instead simply wrap callbacks into our wrapper to establish
the necessary synchronization.
Fixes issue https://code.google.com/p/thread-sanitizer/issues/detail?id=80



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@219675 91177308-0d34-0410-b5e6-96231b3b80d8